This invention relates to the field of grinding, and more specifically to a new and more convenient method and means for securing liners in the large bar mills and ball mills used to comminute ore in commercial mining operations.
A mill of this sort comprises an enormous drum or hollow cylinder mounted on bearings for rotation about a substantially horizontal axis and driven by a very powerful motor through conventional reduction gearing. The ends of such a mill are hollow: material to be comminuted is continuously fed into the mill at one end and the comminuted product continuously emerges at the other end.
Naturally, it is important to keep the mill in operation for as extended intervals as is possible between shut downs for maintenance. The ores being comminuted are highly abrasive, however, and for practical service life, it is necessary that the drum be lined with a special sheet of highly abrasion-resistant character, which must also be tough enough to stand the repeated rolling impact of the ore fragments and of the steel bars or balls, loose in the drum, whose impact is added to the autogeneous grinding of the ore itself.
In view of the tremendous size of the mills, it is necessary to form the lining of a plurality of components, each small enough to be handled - that is, to be inserted into the grinder through one of the axial openings, and to be positioned in a desired location therein - with equipment available at the site of the grinder.
End liners are necessary of course, but do not comprise the subject matter of this application, which relates rather to the lining of the cylindrical surface itself. It has been found that grinder efficiency is improved when the inner surface of the lining is not smooth, but rather is provided with ridges extending axially. A lining is thus constructed of a plurality of liners, or bars, of the special steel extending along the drum. Limitations of size and weight ordinarily do not permit the liners to be of the full length of the drum. These liners, which are subject to the greatest wear and hence most frequently require replacement, are designed to be secured to the inside of the drum by both having their heads received in sockets cast into the steel at known intervals therealong, and passing through holes appropriately located in the shell of the drum, for engagement by nuts extending therethrough.
The securement of the liners without the drum offers certain problems which are not immediately evident. In the first place, the mere size of these mills presents practical difficulties. An illustrative example of such an installation is a ball mill 12 feet long and 28 feet in diameter. In addition to the end liners, 72 rows of liners extend axially within the drum: they are cast from special steel and weigh about 3600 pounds per row. Thus, the drum must support a self-load over 250,000 pounds in addition to the charge of ore and balls or bars, which may add several hundred thousand pounds further. To support so massive a load for rotation at speeds in the neighborhood of 10 revolutions per minute, the drum is formed of steel plates from 1 inch to 11/2 inches in thickness.
The size limit on availability of steel plate, the capacity limit of metal forming machines, and the transportation limits of constructing a mill capable of being shipped from the factory to its remote user, combine to dictate that such a drum may not be unitary. The foregoing mill may be considered exemplary: it is built in two axial sections, each made up of cylindrical quadrants of the chosen axial dimension. After the quadrants are rolled to the desired curvature and axial side flanges and quadrantal curved flanges are welded thereto, each section is positioned on a boring mill to be machined for truing the arcuate flanges and for drilling angularly spaced rows of mounting holes for the linear mounting bolts, at axial intervals equal to those between the sockets in the liners. The same process is repeated for as many cylindrical quadrants as are required to make up the desired length of drum, each section being trued and drilled separately. The flanges are provided with aligned bolt holes for use in assembling the components into a unitary structure.
When the components making up the drum are received at the site where it is to be used, the quadrants are assembled by bolts along the axial flanges to form cylinders, and the cylinders with end plates as necessary are assembled by bolts along circumferential flanges to make up the drum, after which the liners are to be inserted. The circumferential joints along the lengths of the drums are recognizable weaknesses in the complete structure, and it would be desirable to compensate therefor by arranging at least some of the liners to bridge the joints and to be secured within the drum on both sides of the joints. The hole spacing in the drum is determined by the spacing of sockets in the liners, which in casting can be held to an acceptable tolerance. It has been found in practice, however, that while the tolerances for axial spacing of mounting holes in any one cylindrical section of the drum are within practically acceptable limits, the spacing between adjacent mounting holes in axial line but on opposite sides of a circumferential joint cannot be maintained within such limits with the presently available technology. Of course, if one hole across the joint is out of position, any others of that liner on that side of the joint are also out of position by the same amount, within accepted tolerances. This means that no particular trouble is to be anticipated in fastening a liner to the drum as long as it does not extend on both sides of a circumferential joint. If it does so extend, the sockets in the liner may be aligned readily with the holes in the drum on either one side of the joint or the other, but not with holes in both sides of the joint at the same time. It is accordingly been the custom to design the pattern of liners so that no liner extends across a circumferential joint. This obviates the difficulty of hole and socket alignment, but the liners give no reinforcement to the drum itself at the important joint areas.
Some attempts to provide for this situation have been made. The bolt holes are usually made larger in diameter than the bolts, which allows for a certain amount of linear shifting of the bolt in the hole and socket, and which permits a certain degree of cocking of the bolts as they pass through the drum. Cocking is undesirable as it not only tends to sheer the parts as they are driven together, but also causes "ovaling" of the mounting holes in the drum, as well as quickly abrading the bolt shanks just under the heads, and results moreover in undesirable stress distribution in the drum, the casting, and the bolts.
There have also been suggested certain particular configurations of bolt heads and liner sockets which are intended to give some degree of freedom of angulation of the bolt axis. These expedients have proved helpful for minor out-of-tolerances such as a cure in the circumferential alignment of the bars and holes, but not for such major discrepancies as unavoidably occur in the axial direction.
It is not practical to drill out any holes in the liners at the time of erection as by their very nature the bars are extremely resistant to the abrasion of cutting tools. Moreover, because of their composition and hardness, the liners cannot be cut acceptably with torches. On the other hand, relocation and reboring of holes in the drum on the site, with the repeated insertion and removal of the massive liners for trial purposes which is incidental thereto, is an intolerably expensive and arduous process.